In order to provide complete suppression of voltage transients in electrical circuits, and particularly circuits that include hot, neutral, and ground conductors, it is desirable to provide surge suppression utilizing such components as metal oxide varistors, bridging each of three possible pairs of conductors in which these voltage transients can occur. More specifically, it is desirable to provide a surge protection device connected between the hot and ground lines, between the hot and neutral lines and between the neutral and ground lines.
Some forms of TVSS devices, such as varistors, are subject to failure during operation, either as a result of high energy transient voltages being applied to the devices, the repetitive application of low energy transient voltages being applied to the devices, or even the long term application of steady state voltages above the suppression threshold. Such varistors are subject to failure as a result of thermal or chemical breakdown of the components of the varistor. Any of these may lead to immediate varistor failure.
As the varistor fails or begins to fail, it's surface can begin to break down, become conductive and cause short-circuiting with other conductive parts of the TVSS product. As a result, the electronic components and other metal components must be shielded from each other and/or separated by distances large enough to prevent short circuiting and premature failure of the device. Such separation distances may limit the size and number of components that can be used in a receptacle with conventional dimensions.
The failure mode associated with metal oxide varistors, produces a short circuit or low impedance failure. That is, the impedance of the device is reduced significantly below its normal operating impedance, as a result of failure. It is known to protect electrical circuits against such failure, by providing fuses in series with the varistors so that if the varistors fail, the fuses open and the varistor does not itself cause a short circuit or low impedance condition on the power circuit, which could create a risk of overloading the circuit, leading to overheating, the tripping of remote circuit breakers, or the like.
When a metal oxide varistor fails in a way that causes a series connected fuse to open, the protection provided by the varistor is lost. Moreover, the failure may be undetectable by observation of the devices connected to the circuit. Therefore it is possible for the transient voltage protection to be lost without any obvious signs thereof, and thereafter for voltage transients to be passed unsuppressed to the equipment connected to the previously protected circuit, possibly causing damage.
In an effort to provide some indication of the failure of certain suppression devices, particularly varistors, indicators have been provided. Such indicators may be visual, such as a light emitting diode or other visible device; or audible such as a buzzer, horn or the like.
While audible failure indicators are particularly desirable in applications where the TVSS device is mounted in a visually inaccessible area, an audible alarm can itself cause problems. In applications where a fault occurs and an audible alarm sounds, it may be some time before a repair can be effected. Since surge suppression devices are often combined with electrical receptacles which are permanently connected to electrical circuits, trained personnel are needed to effect a replacement in the event of a fault.
In residential settings, or even commercial or industrial settings, a trained person is not always immediately available. In these cases, the incessant buzzing of an audible alarm may be distressing to residents forced to listen to it during the time it takes for the device to be replaced. Since a control for deactivating a buzzer must be accessible on the outside of the device, it may happen that the control is either accidentally or even maliciously engaged before a fault occurs, thereby in some instances rendering the audible portion of the alarm ineffective in the event of a failure. It would be advantageous to provide a control for disengaging an audible alarm that can be switched from an engaged position to a disengaged position repeatedly, so that if the alarm is accidentally disengaged before a fault occurs, the device is not thereby rendered inoperative in the future. It is desirable to do this switching electrically rather than mechanically, so as to provide repeatable engagement and disengagement without damage as may occur with a mechanical approach.
Known switches having metal contacts which are repeatedly engaged and disengaged may suffer from metal fatigue and permanent deformation due to over travel of the metal switch blade. This may result in decreased reliability and ultimate failure of the switch.
It is another more specific object of the invention to provide a TVSS switch with increased reliability having a switch blade with an integral stop for preventing overtravel of the switch blade and preventing permanent deformation of the switch blade.